Arrangement and method for operating a variable drive unit

ABSTRACT

Arrangement and method for operating a drive unit for motor vehicles, particularly for agricultural and construction machinery, with a controllable internal combustion engine (2), a continuously variable hydromechanical power split transmission (6) and a control device (3) which in order to start and to operate the vehicle at low speeds and under load, at first reduces the rotational speed of the internal combustion engine to less then the nominal speed, reaches a first speed by reducing the delivery volume of the adjusting pump (13) of the hydrostatic branch (15), a further increased speed by increasing the rotational speed of the internal combustion engine and as a result the further increase in the vehicle speed by controlling the transmission ratio of the hydrostatic-mechanical power split transmission (6) is achieved.

The invention relates to an arrangement and a method of operating avariable drive unit for a motor vehicle, particularly for agriculturaland construction machinery., with a controllable internal combustionengine and a hydrostatic-mechanical power split transmission withcontinuously variable transmission ratio according to the introductorypart of claim 1.

From the patent specification (DE 35 12 523) a continuously variablepower split compound load-shifting transmission with group shifting isknown. The transmission consists of two gear subgroups. One of them is acontinuously variable power split linkage with several shafts. The othergear subgroup is a gear-shift mechanism with one speed group and severalauxiliary gears which act upon the main driven shaft. The two linkageshafts alternately actuate the gear-shift mechanism. The gear shiftsoccur free of load at synchronous rotational speeds and withoutinterruptions of traction power. The transmission works like acontinuous transmission with a large range of controller output andmakes possible to operate the internal combustion engine of a vehiclealong a characteristic curve for minimal fuel consumption, this wayachieving fuel economy.

From the laid open FR-A 21 43 154 a continuously variablehydromechanical load-shifting transmission is known, which can beadjusted in such a manner that in all operational states the drivinginternal combustion engine is operated in its best performance range. Inorder to avoid skidding of the vehicle tiers when the engine developsits full power, in the transmission for instance a pressure-controlvalve is provided in the hydrostatic transmission branch, in order tolimit the transmittable maximum torque. This hydromechanicalload-shifting transmission is purely hydrostatically operated for thestart and reverse drive.

Drive units with hydrostatic-mechanical power split transmissions formotor vehicles usually register very high losses under certainoperational conditions of the vehicle, e.g. at start or while travelingwith very low speeds under high load. When the vehicle is standing andshifted into the first speed range, the primary unit built like anadjusting pump reaches its maximum delivery volume already in idling atthe nominal speed of the internal combustion engine, whereby highhydrostatic idling losses result. With increasing load at the start, thedelivery volume of the adjusting pump decreases, however, due to theload-dependent pressure increase, the hydraulic losses remainconsiderable, so that in this operational range the general efficiencyremains low. Further, the large delivery volumes of the adjusting pumpat high pressures in correlation with the high rotational speedstransmitted to the hydrostat by an internal combustion engine at nominalspeeds leads to premature wear and to high noise emissions of thetransmission.

For motor vehicles which start frequently from stillstand and/orpermanently operate under conditions of very low speeds and high loads,such as agricultural and construction equipment, the operation ofhydrostatic-mechanical power split transmissions is particularlyproblematic.

It is the object of the invention to improve on an arrangement and amethod for operating a variable drive unit with a controllable internalcombustion engine and a hydrostatic-mechanical power split transmissionso that the motor vehicles which have to start frequently and/or travelat very low speeds under high load, can be operated with increasedefficiency of the transmission and with reduced wear, reduced noiseemission and lower speed (rpm) of the internal combustion engine.

The problem is solved according to the invention in that a commoncontrol device for the internal combustion engine and the power splittransmission is provided, device which in a first phase of a startingoperation under full load sets the speed n_(mot) of the internalcombustion engine to a lower value than the nominal rotational speed,resets the adjusting pump to a maximal delivery volume and engages theclutch for the first speed range of the power split transmission, inthat during a second phase of the starting process, the delivery volumeof the adjusting pump is reduced by the control device at a loweredrotational speed n_(mot) until the hydromotor stops, in that in a thirdphase of the starting operation the vehicle speed with the hydromotor ata stillstand is increased by raising the rotational speed n_(mot) to thenominal speed and that subsequently the speed increases up to the targetspeed solely due to the change of the delivery volume of the adjustingpump, respectively the gear-range shifting of the clutches.

This arrangement and this method for operating a continuously variabledrive unit with a controllable internal combustion engine and ahydrostatic-mechanical power split transmission for a motor vehicle hasthe advantages that the power of the internal combustion engine isbetter attuned to the transmission ratio of the power split transmissionat the start point and at very low speeds under high load, so that thetraction of the motor vehicle is optimally exploited, the power flow inthe hydrostatic transmission branch is diminished and the generalefficiency of the power split transmission is improved. The constructionof the power split transmission is not changed as a result of thisimprovement.

Theoretically, hydrostatic-mechanical power split transmissions have aninfinite transmission ratio and offer correspondingly high torques atthe drive axles at the start and at low speeds. However, the traction ofa motor vehicle can transmit only a limited torque at start or at lowspeeds. The available torque of the internal combustion engine at theinput of the hydrostatic-mechanical power split transmission cantherefore be reduced , without creating disadvantages at the start or atlow vehicle speeds. In addition, due to the invention, the strain andthe wear of the internal combustion engine can be reduced.

According to claim 2, the control device can advantageously limit thespeed of the internal combustion engine to maximum one half of thenominal speed. Of course, thereby it has to be avoided that it fallsbelow the idling speed of the internal combustion engine.

The invention is not limited to the combination of the features in theclaims. For the person skilled in the art, further combinations arepossible without further ado within the framework of the invention.

FIG. 1 shows the arrangement according to the invention

FIG. 2 shows in a diagram the rotational speed ratios and the pressurecurve of a continuously variable drive unit, consisting of acontrollable internal combustion engine and a hydrostatic-mechanicalpower split transmission according to the state of the art.

FIG. 3 shows in a diagram the rotational speed ratios and the pressurecurve of a continuously variable drive unit, consisting of acontrollable internal combustion engine and a hydrostatic-mechanicalpower split transmission as proposed by the invention.

FIG. 1: At a selector switch 1 a freely selectable speed is set. At theselector switch 1, it is possible to select an economy-program for theoperation of the internal combustion engine 2 at low consumption valuesand a full-load program for operating the internal combustion engine 2at nominal (rated) speed. A control device 3 exchanges electricalsignals, e.g. command signals and feedback, with the selector switch 1,via lines 4, 5. A variable drive unit consists of a controllableinternal combustion engine 2 of the usual construction, which drives ahydrostatic-mechanical power split transmission 6. Between the internalcombustion engine 2 and the power split transmission 6, there is noshiftable clutch. The control device 3 of known construction type, forinstance an electronic control device, automatically controls the speedof the internal combustion engine 2 and the transmission ratio of thecontinuously variable hydrostatic-mechanical power split transmission 6,so that the preselected speed is initiated. For this purpose, measuredvalue sensors which are not shown in the drawing are provided on arotational-speed dependent driven input shaft 7 and on a transmissionoutput shaft 8, which transmit the rotational speeds of the shafts tothe control device 3 over lines 9, 10. The control device 3 compares themeasured rpm values with the command signals from selector switch 1.Depending on the drive program stored in the control device 3, controlsignals are sent from the control device 3 over lines 11, 12 to theinternal combustion engine 2 and the power split transmission 6.

The control of the internal combustion engine 2 takes place in the usualmanner by throttling the air-suction cross section or by adjusting thefuel injection.

The transmission ratio of the hydrostatic-mechanical power splittransmission can be continuously varied, by continuously adjusting thedelivery volume of a primary unit 13 and thereby the rotational speed ofa secondary unit 14-- built as a hydromotor-- of the hydrostatic branch15 of the transmission 6. The primary unit 13 in the hydrostatictransmission branch 15, which is designed like an adjusting pump, isdirectly driven by the controllable internal combustion engine 2 via adrive shaft 7, and rotates with a rotational speed proportional to thespeed of the internal combustion engine 2. The continuous transmissionrange in the hydrostatic transmission branch 15 in the power splittransmission 6 is multiplied in the mechanical branch 16 by gearelements, e.g. planetary gears.

Over a reduction gear 19 and a shaft 20, the internal combustion engine2 drives the mechanical transmission branch 16, which comprises at leastone linkage gear (not shown). Linkage shafts 21, 22 of the linkage gearare driven with continuously variable rotational speeds, which resultfrom the summation of the continuously variable rotational speeds of thehydromotor 14 and the constant rotational speed of the shaft 20.

The clutches 23 alternately connect one of the shafts 21 or 22 with thetransmission output shaft 8, which drives the rear axle 24.

DESCRIPTION OF OPERATION

FIG. 2: According to the state of the art, in a continuously variabledrive unit under full load the start takes place from the stillstand atnominal speed of the internal combustion engine 2 (line L1). The speedof the internal combustion engine 2 remains constant over the entirespeed range. The adjusting pump 13 is directly driven by the internalcombustion engine 2 and rotates with a speed which is proportional tothe nominal speed of the internal combustion engine 2. Line L2 shows thecurve of the rotational speed of hydromotor 14 and line L3 thecharacteristic curve of the pressure in the hydrostat 15 over speed withits absolute maximum in the starting range.

FIG. 3: Point P1: In order to start the motor vehicle under full load,according to the invention the control device 3 at first reduces thenominal speed of the internal combustion engine 2, so that the internalcombustion engine 2 runs with a lower speed (rpm) or at most with halfthe nominal speed. The adjusting pump 13 in the hydrostat 15 turns thenalso with half of the nominal rotational speed and drives the hydromotor14 with increasing delivery volume. When the hydromotor 14 has reachedin the hydrostat 15 a rotational speed , whose summation with a constantrpm in a linkage gear (not shown) results in a rpm equalling zero at thelinkage shaft 21 or 22 pertaining to the first speed range, the controldevice 3 actuates the clutch 23 for the first shift range of themechanical transmission branch 16. At this point, at the output shaft 8of the power split transmission the rotational speed still equals zero.

P1-P2: The control device 3 acts upon the adjusting device 25 andreduces the delivery volume of adjusting pump 13. The rotational speedof the hydromotor 14 (line 2) decreases. The rotational speed of theshaft 20 of the mechanical transmission branch 16 remains proportionalto the constant speed of the internal combustion engine 2. The summationof the decreasing and the constant rotational speed in the linkage gearresults in a rotational speed difference, the transmission output shaft8 starts rotating and the vehicle is set in motion.

Point P2: The adjusting pump 13 delivers only the amount which isnecessary to keep the hydromotor 14 at a rotational speed equal to zeroand to compensate leak losses.

P2-P3: The further increase in the vehicle speed takes place byincreasing the rotational speed of the internal combustion engine 2(line L1) up to the nominal rpm. The rotational speed of the adjustingpump 13 and of the mechanical branch 16 of the power split transmission6 increases proportionally to the speed of the internal combustionengine 2. The delivery volume of the adjusting pump 13 remainsconstantly low (line L2). The rotational speed of the hydromotor 14continues to be zero. The power flow through the hydrostat is low andthe transmission runs at a favorable efficiency degree.

From point P3: The speed is further increased by further resetting theadjusting pump 13 to a larger delivery volume, so that the hydromotor 14is again accelerated (line L2). The internal combustion engine remainsconstant at the nominal rotational speed (line L1). The preselectedspeed of the vehicle is reached by gear-shifting in the mechanicaltransmission branch 16 and changes in the delivery volume in thehydrostatic transmission branch 15 of the power split transmission 6.

Preferably, in order to start the vehicle under load, the rpm of theinternal combustion engine 2 is adjusted at stillstand of the vehicle tohalf of the nominal engine speed and then continues to maintain this rpm, while the adjusting pump 13 reduces the delivery volume. At stillstandof the hydromotor 14 in the hydrostat 15, the further speed increase ofthe vehicle has then to take place through an increase in the rotationalspeed of the internal combustion engine 2 until it reaches the nominalrpm, and after that a new speed increase of the vehicle takes placeagain by readjusting the power split transmission 6, until thepreselected speed is reached.

Within the framework of the invention it is also possible to conceive adifferent division of the part played by the change in the rotationalspeed of the internal combustion engine 2 in the increase of the vehiclespeed at start under full load.

    ______________________________________                                        Reference Numerals                                                            ______________________________________                                         1      selector switch                                                        2      internal combustion engine                                             3      control device                                                         4      line                                                                   5      line                                                                   6      power split transmission                                               7      shaft                                                                  8      transmission output shaft                                              9      line                                                                  10      line                                                                  11      line                                                                  12      line                                                                  13      adjusting pump                                                        14      hydromotor                                                            15      hydrostatic transmission branch                                       16      mechanical transmission branch                                        17      reduction gear                                                        18      hollow shaft                                                          19      reduction gear                                                        20      shaft                                                                 21      linkage shaft                                                         22      linkage shaft                                                         23      clutch                                                                24      rear axle                                                             n.sub.mot                                                                             rotational speed of the internal combustion engine                    line L1 characteristic curve of the speed of the                                      internal combustion engine over the vehicle speed                     line L2 rpm of the hydromotor over vehicle speed                              line L3 characteristic curve of the hydrostat pressure                                over vehicle speed                                                    point P3                                                                              starting point                                                        P1-P2   reduction of the delivery volume at constant                                  rpm of the internal combustion engine                                 P2-P3   rpm of the internal combustion engine at                                      constant delivery volume                                              ______________________________________                                    

We claim:
 1. Method of operating a drive unit for motor vehicles,particularly for agricultural and construction machinery, consisting ofan internal combustion engine with a controllable engine speed n_(mot)and a continuously variable hydrostatic-mechanical power splittransmission, whose multi-shaft linkage gear and whose regulatingtransmission having an adjusting pump are driven by the internalcombustion engine, whereby a rotational speed of a hydromotor of theregulating transmission is superimposed on the linkage gear and wherebythe linkage shafts of the linkage gear, which have synchronousrotational speeds at points close to the gear ranges of the regulatingtransmission, are alternately connectable with an output shaft overgear-shift clutches, characterized in that in a first phase of thestarting operation under full load, the engine speed n_(mot) is set at avalue below the nominal rotational speed, the adjusting pump (13) isreset on maximum delivery volume and the clutch (23) is engaged for thefirst speed range of the power split transmission (6), that during asecond phase (P114 P2) of the starting process the delivery volume ofthe adjusting pump (13) is lowered at a reduced engine speed n_(mot),until the hydromotor (14) is stopped, that in a third phase of thestarting process (P2-P3) the vehicle speed, with stopped hydromotor(14), is increased by raising the engine speed n_(mot) to the nominal(rated) speed and that subsequently the speed increases until it reachesthe preselected target speed, solely due to the change in the deliveryvolume of the adjusting pump (13), respectively the gear-range shift ofthe clutches (23).
 2. Method of operating a drive unit for motorvehicles according to claim 1, characterized in that at the start of thevehicle under full load, the speed n_(mot) of the internal combustionengine is first limited to at most one half of the nominal speed, underthe assumption that it does not thereby fall below the idling speed. 3.Arrangement for operating a drive unit for motor vehicles, particularlyfor agricultural and construction machinery, consisting of an internalcombustion engine with a controllable speed n_(mot) and a continuouslyvariable hydrostatic-mechanical power split transmission, whosemultishaft linkage gear and whose regulating transmission which has anadjusting pump are driven by the internal combustion engine, whereby arotational speed of a hydromotor of the regulating transmission issuperimposed on the linkage gear and whereby the linkage shafts of thelinkage gear, which have synchronous rotational speeds at points closeto the gear ranges of the gear-range limits of the regulatingtransmission, are alternately connectable with an output shaft overgear-shift clutches, characterized in that a common control device (3)is provided for the internal combustion engine (2) and the power splittransmission (6), which in a first phase of the starting operation underfull load sets the engine speed n_(mot) at a value below the nominalrotational speed, resets the adjusting pump (13) on maximum deliveryvolume and engages the clutch (23) for the first speed range of thepower split transmission (6), that during a second phase (P1-P2) of thestarting process the delivery volume of the adjusting pump (13) islowered by the control device (3) at a reduced engine speed n_(mot),until the hydromotor (14) is stopped, that in a third phase of thestarting process (P2-P3) the vehicle speed, with stopped hydromotor (14)is increased by raising the engine speed n_(mot) to the nominal (rated)speed and that subsequently the speed increases until it reaches thepreselected target speed, solely due to the change in delivery volume ofthe adjusting pump (13), respectively the gear-range shift of theclutches (23).
 4. Arrangement for operating a drive unit for motorvehicles according to claim 3, characterized in that at the start of thevehicle under full load, the control device (3) first limits the speedn_(mot) of the internal combustion engine to maximum one half of thenominal speed, under the assumption that thereby it does not fall belowthe idling speed.